Broad band antennas having spiral windings

ABSTRACT

A broad band antenna or feed for a parabolic reflector having a pair of spiral windings skewed with respect to the axis of a conical support surface on which they are wound in alternating relationship. Two such supports are rotatably mounted with their axes offset at an angle from the reflector axis so as to conically scan the target area. The feed achieves maximum illumination of the control portion of the reflector and provides a constant squint angle with respect to the reflector axis over a broad band of frequencies.

United States Patent [72] inventors James W. Crooks, ,Ir.;

Ray M. McIntyre, Jr., both of San Diego, Calif. [21] Appl. No. 745,014[22] Filed July 15, 1968 [45} Patented June 22, 1971 [73] AssigneeGeneral Dynamics Corporation [54] BROAD BAND ANTENNAS HAVING SPIRALWINDINGS 17 Claims, 7 Drawing Figs.

[52] US. Cl 343/739, 343/761, 343/840, 343/895 [51] Int. Cl l-l0lq H36[50] Field of Search 343/7925, 840. 895. 761.739

[56] References Cited UNITED STATES PATENTS 2,958,081 10/1960 Dyson343/895 2,982,964 5/1961 Bresk et a1. 343/895 3,059,234 10/1962 Du Hamelel 211... 343/7925 3,193,831 7/1965 Yang 343/840 3,277,490 9/1966Williams 343/840 3,454,951 7/1969 Patterson et al. 343/895 2,682,6086/1954 Johnson 343/895 2,966,679 12/1960 Harris 343/895 PrimaryExaminer-Eli Leiberman AttorneyM artin Lu Kacher ABSTRACT: A broad bandantenna or feed for a parabolic reflector having a pair of spiralwindings skewed with respect to the axis of a conical support surface onwhich they are wound in alternating relationship. Two such supports arerotatably mounted with their axes offset at an angle from the reflectoraxis so as to conically scan the target area. The feed achieves maximumillumination of the control portion of the reflector and provides aconstant squint angle with respect to the reflector axis over a broadband of frequencies.

SUPPORTING CONE OF INSULATING MATERIAL PATENIEII JUII22 IBYI SHEET 1 [IF5 fPHASE FRONT DISH I 2 TYPICAL E PLANE RADIATION PATTERN 26 OFCONVENTIONAL FEED e EFFECTIVE CENTER OF RADIATION FOR LOW FREQUENCYCENTER OF I2 CONVENTIONAL RADIATION I ,RcP FEED AT SHORT AXIS 0F FEEDROTATION 4 WAVE E AND AXIS OF DISH I DESIRED scum-r ANGLE DESIRED BEAMANGLE OF MAJOR ILLUMINATION BY I FOCAL D'RECT'ON FEED 95}? l4 FEED DISHPERPENDICULAR TO PORTION OF PHASE FRONT ILLUMINATED I ACTUAL SOUINTANGLE O ILLUMINATED PORTION OF PHASE FRONT Fig.

INVI'IN'H )R. JAMS w. (woo/(s, JR. RA) M. Mel/VT YRE, JR.

RYm JBQ ATTY PATENTEUJUNZZIB'II 3587.106

SHEET 2 BF 5 DISH L-PHASE FRONT TYPICAL E PLANE RADIATION PATTERN OFFEED WITH SKEWED WINDING EFFECTIVE CENTER OF RADIATION FOR LOW FREQUENCYI RCP FEED WITH SKEWED WINDINGS SQUINT ANGLE ROTATION BEAM DIRECTION LCPFEED WITH SKEWED WINDINGS ANGLE 0F PRINCIPAL ILLUMINATION INVENTORS.

JAMES W CROOKS, JR. RAY M Mc/NTYRE, JR.

BY ATTY.

slsamos PATENTED .JUN22 197i SHEET 3 BF 5 IN VIE N TOR 8 JAMES w. moons,JR. RA r M. Mc/N TYRE, JR

PATENTEUJUN22|9H 3587.106

SHEET 0F 5 SKEW ANGLE INVENTOR. JAMES w. moo/rs, .m. RAY M. m INTYRE,JR.

PATENTED JUN22 l9?! SHEET 5 0F 5 I N VENTOR.

JA MES W. CRO0K$,JR.

RAY M. MCINTYREMR.

BY Qtfwh v ATTY BROAD BAND ANTENNAS HAVING SPIRAL WINDINGS The presentinvention relates to broad band antennas and particularly to broad bandantennas having conical spiral feed windings.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 ofNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 USC 2457).

The invention is especially suitable for use in a feed for parabolicantennas which are adapted to track space vehicles by responding tosignals radiated therefrom. The invention however is also generallyuseful in broad band antennas, including antennas of a type which do nothave a reflector dish as well as in directional antennas where a desiredradiation pattern in a direction having a specified angle with respectto the angle at which the antenna is mounted may be desired.

The conventional conical antenna having equiangular spiral windings hasa radiation pattern which is directed along the axis of the cone onwhich the windings are disposed. It has been found that thischaracteristic is detrimental to a scanning antenna where the axis ofthe cone is offset from and makes an angle with the axis of thereflector with which the antenna is used as a feed. The term feedcomprehends both reception and transmission of signals.

At certain lower frequencies in the band in which the antenna is activealthough the feed is positioned for proper operation at the highestfrequency, instead of scanning conically in synchronismwith the rotationof the feed, the radiation pattern from the reflector remainsessentially along the reflector axis. This effect is manifestedparticularly in the case of the plane of the Electric Vector of linearpolarized radiation. Since the antenna does not scan with the requisitepattern on which the servosystem of the tracking system depends, itbecomes difficult to lock on to and to track the source of incomingradiation, say a space vehicle or missile. In the event that the ratioof focal length with respect to the diameter of the reflector is smallas for example (in deep parabolic dishes) the side lobe energy or sidelobe radiation pattern of the feed impinges on the reflector and thefeed pattern does not effectively illuminate the whole reflectingsurface of the dish thereby compromising antenna performance.

It is therefore an object of the present invention to provide improvedscanning antennas.

It is a further object of the present invention to provide improvedbroadband antennas.

It is a still further object of the present invention to provide animproved conical feed for spiral antenna.

It is a still further object of the present invention to provide awideband conical spiral type feed which may be used in combination witha dish or by itself as an independent antenna and whereby the radiationpattern of the feed remains at a desired angle with respect to the axisthereof over the entire band of frequencies over which the antenna isactive. The angle can be made approximately constant or varied over thefrequency range of the antenna by controlling the angle of skew alongthe winding.

Briefly described, an antenna or feed embodying the invention includes asupport structure. A winding is spirally wound around the supportstructure to define a substantially conical surface or cone. A portionof the windings can be disposed at a predetermined angle with respect tothe axis of the cone so that the direction of the radiation patternremains at a constant angle with respect to the'cone axis. In the eventthat the aforementioned antenna structure is sued as a feed for anantenna having a reflector, it is desirably supported so that the coneaxis is at a predetermined angle to the axis of the reflector. Thisangle being selected to achieve a desired squint angle. The squint angleof the antenna, when this invention is used as a feed in a reflector, isthe direction of the radiation pattern from the reflector with respectto the reflector axis. By virtue of the skewed windings on the feed, thesquint angle of the antenna can be made to be approximately proportionalto the antenna beam width over the entire wide band of frequencies overwhich the antenna is active, notwithstanding that the feed may berotated about the axis of the dish in order to pro vide scanning action.

The invention itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof willbecome more readily apparent from a reading of the following descriptionin connection with the accompanying drawings in which:

FIG. I is a schematic diagram of a conventional antenna of the typehaving a dish illuminated by a feed on which equiangular conical spiralwindings are disposed;

FIG. 2 is a schematic diagram similar to FIG. 1 of an antenna whichembodies the invention:

FIG. 3 is a plan view of a conventional equiangular or logarithmicconical feed or antenna;

FIG. 4 is a view similar to FIG. 3 of an antenna or feed which embodiesthe invention;

FIG. 5 is a sectional view taken along the line 5-5 in FIG. 4;

FIG. 6 is a section view taken along the line 6-6 in FIG. 4; and

FIG. 7 is a plan view of a conical spiral antenna or feed in accordancewith another embodiment of the invention.

FIG. 1 illustrates a conventional telemetry antenna which may be used asa receiving antenna for tracking and receiving signals from spacevehicles and missiles. The antenna may also be used for transmitting.The antenna includes a dish reflector which is parabolic in form and hasa focal point along the axis thereof. The dish is relatively deep, thatis to say the ratio of its focal distance (viz. the distance from thefocal point of the dish to its surface (F) to the diameter of the dish(d)) is relatively low say of the order of 0.3. The antenna includes afeed 17 which is shown in greater detail in FIG. 3. This feed includestwo conical members 12 and 14, which may be made of insulating material.Each of these members carries a pair of windings 16a and 16b and 18a and18b. The windings 16a and 16b on the conical member I2 alternate and arewound in the form of equiangular spirals. Windings 18a and 18b on theconical member 14 are similarly wound as equiangular spiral windings butin a sense opposite to the windings 16a and 16b. Thus the windings 16aand 16b will be responsive to one sense of circular polarized radiationwhereas the windings 18a and llfib will be responsive to the oppositesense of circular polarized radiation. The conical members are supportedon a yoke 20 which may be joumaled for rotation about the axis of thedish.

Transmission lines 19 and 21 are respectively provided in thecylindrical members 12 and I4 and are connected at their front ends tothe ends of the windings near the apex of their respective conicalmembers. The other end of the windings I6 and 18 are terminated, say byresistors 22 and 24, respectively, each being connected between theopposite ends of their associated windings. The ends of the transmissionlines 19 and 21, which extend from the base of the conical members maybe connected by way of cables to a rotating joint for deriving signalsform the respective windings I6 and 18. A two wire transmission line maybe used for 19 and 21 or alternately a coaxial line with a balun tocouple from the coaxial line to the balanced transmission line at thetip of the cone.

Altemately a coaxial transmission line can be used as the spiral windingwith a coaxial transmission line bringing the signal in (or out)connected to one of the two spirals in the place of the resistorterminatiomfl" he coaxial transmission line center conductor can beconnected from one spiral to the center conductor of the opposite spiralon the same cone at the small end (apex) of the spiral with a small gapbetween the outer conductor of one winding and the outer conductor ofthe second winding. The outer conductor of the second spiral winding isshorter to the inner conductor at the end of the gap. This is aconventional method of feeding equal angle spiral antennas from acoaxial transmission line and is sometimes referred to as the use of aninfinite balun. The terminating resistor is optional. The antenna, byvirtue of the fact that the windings I6 and 18 have smaller diameterportions which spiral outwardly to larger diameter portions is operativeover a broad band of frequencies, say from the S band through the UHFband. The effective center of radiation (viz. the point on the feed 17where the antenna is responsive to different frequencies of radiation)varies with the distance along the axes of the conical members 12 and14. The shorter wavelengths will have their centers of radiation nearthe apex of the cone and for longer wavelengths the center of radiationwill move along the axis of the conical members away from the apexthereof.

As shown in FIG. 1, the apex of the conical members and the center ofradiation at short wavelengths is close to the focal point of the dish.A typical E-plane radiation pattern at lower frequencies and longerwavelengths is also shown spaced at its center further away from thedish. In order that the antenna be responsive to signals at the longerwavelengths, it is necessary that the antennas scan a relatively widefield of view at lower frequencies. Such a wide scanning field dictatesa relatively large squint angle. By a squint angle is also meant theangle between the axis of the dish and the beam direction. The desiredsquint angle and the desired beam direction is shown in FIG. 1. In orderto obtain this larger squint angle at longer wavelengths, it becomesnecessary to offset the axis of the conical members from the axis of thedish. The foregoing offset is dictated by the characteristic ofequiangular spiral antennas of providing a radiation pattern which isalong the axis of the conical members, as shown in FIG. 1. By virtue ofthis offset the angle of major illumination of the dish at lowerwavelengths is not sufficiently broad to evenly illuminate the fulldish, especially in the plane of the Electric Vector (E- plane). ln deepparabolas, this results in side lobe energy impinging on the reflectorsurface; further, compromising antenna performance. ln addition, theeffective squint angle may be actually reduced to zero at some wavelengths. This characteristic follows from the fact that the actualdirection of the radiation (viz. the direction of the beam) isperpendicular to the phase front from the portion of the dish which isilluminated. The phase front is the surface along which all of theenergy is in phase. The phase front of the radiation from theilluminated portion of the dish at any frequency is defined by a surfacegenerated by the continum of points having equal distances along raypaths from the center of radiation of waves at that frequency to thedish and then outwardly from said dish. Two of such points 26 and 28 atthe limits of the angle of major illumination of the dish at a longerwave length in the UHF band are shown in FIG. 1. These points aregenerated by drawing ray paths from the center of radiation along astraight line to the dish and then from the dish such that these anglesof incidence 6, and 6, equal these angles of reflection 6, and 0,,respectively. The distance along the line between the center ofradiation to the dish and thence to the point 26 on the phase front isequal to the distance along the lines from the center of radiation tothe dish and thence to the point 28. Since the actual direction of thebeam or the direction of the radiation pattern of the antenna isperpendicular to the phase front which is illuminated, it may bedetermined by construction that the actual direction of the beam isapproximately parallel to the axis of the dish making the actual squintangle approach zero degrees (i.e. along the dish axis) for radiation oflower frequency and longer wave lengths.

in accordance with the invention this deficiency is overcome byproviding a radiation pattern from the feed which is essentially in adirection parallel to the axis of the dish so that the dish isilluminated throughout the frequency range over which the antenna isactive. FIG. 2 shows a dish 30 which may be a relatively deep parabolicof the type shown in FIG. 1. The antenna includes a feed 32 includingtwo conical members 34 and 36. The conical member 34 having windingswound thereon in one sense to provide a left circularly polarized feedelement whereas the conical member 36 has windings wound thereon in anopposite sense to provide a right circularly polarized feed. Thewindings are skewed with respect to the axis of their respective conicalmembers 34 and 36. The resulting E-plane radiation pattern iseffectively parallel to the axis of the dish so that the angle ofprincipal illumination of dish 30 is distributed over approximately thecentral portion of the dish. The phase front resulting from thisillumination may be determined by construction of the same manner asexplained in connection with FIG. 1. lt will be observed that two points38 and 40 on the phase front along rays at the edges of the angle ofprincipal illumination are approximately at the same distance from thedish axis.

Inasmuch as the actual beam direction is perpendicular to the phasefront from the illuminated portion it is apparent that the beamdirection will be at the desired squint angle and radiation at longerwavelengths may readily be received by the antenna as it scans a widerarea. Accordingly, the invention provides a broad band scanning antennawith adequate squint in that it makes possible adequate scanning atwavelengths at which the squint is inadequate with conventional feed toproduce adequate tracking error signals for proper servo performance. Asshown in FlGS. 4, 5 and 6 the conical members 34 and 36 may be hollowcones of fiberglass or other insulating material on which pairs ofwindings 40 and 42 are wound; the windings 40a and 40b being wound onthe cone 34 while the windings 42a and 42b are wound on the cone 36. Thecones are supported on a yoke 44 which may be journaled for rotationabout the axis of the dish, as shown in FIG. 2. The windings 40a and4012 as observed on one side of the conical support alternate with eachother and are connected at their ends near the apex of the conicalmember 34 to a transmission line (not shown) similar to the transmissionline shown in FIG. 3. The windings 42a and 42b are wound in a senseopposite to the windings 40a and 40b and are also alternated with eachother. Another transmission line may be connected to the apex ends ofthe windings 42a and 42b. The other ends of the windings 40a and 40bnear the base of the cone 34 may be terminated by a resistor 46 whilethe ends of the windings 42a and 42b near the base of the cone 36 may beterminated by a resistor 48. This resistive termination is not essentialand may be omitted if desired. A termination is, however, preferred.

In order to enchance the performance of the feed 32, it may be desirableto position a ground plane at the bases of each of the cones 34 and 36.Alternatively, the ends of the windings 40a and 40b and 42a and 42b maybe terminated by rings 50 (FIG. 7) of conductive material such as wirewhich is disposed around cylindrical portions 52 extending from thebases of the cones; only one of the cones is illustrated in FIG. 7 toshow this construction. A first plurality of resistors 54 is connectedbetween the end of the winding 40a and the ring 50 while a secondplurality of resistors 56 is connected between the end of the winding40b and the ring 50. These resistors are parallel to the axis of thecylindrical portion 52 and are spaced from each other. The spacingbetween the resistor 54 is desirably logarithmic and increases betweenresistors which are spaced further away from the end of the winding 40a.A similar space relationship may be provided between the resistors 56which terminate the end of the winding 40b.

The surface of the cones 34 and 36 is of course defined by theconductive elements or windings 40 and 42 thereon. As shown in FlG. 5this surface can be made elliptical in cross section for cross sectionstaken perpendicular to the axis of the cones and the surface be madeconical for cross sections taken perpendicular to the scanning axis(viz. the axis of the dish) as shown in FlG. 6. This is a desirable butnot essential feature of the design. The windings 40 and 42 are alsoskewed with respect to the axis of the respective cones 34 and 36 (viz.the conical surfaces which they defined). As pointed out in connectionwith FIG. 2, it is desirable that the direction of radiation at thelonger wave length be essentially parallel to the axis of the dish. Thisis accomplished by providing the skew of the windings such that thewindings are tilted at a skew angle approximately equal to the anglewhich the axis of the cones make with the dish axis. The skew angle isidentified in FIG. 4. The projection of the windings on a planeperpendicular to the axis of the cones (viz. the conical surfacesdefined by the windings) can be beneficially modified to be anelliptical spiral rather than circular spiral as is the case with anequiangular spiral winding, such as is shown in FIG. 3. This angle ofskew as shown in H6. 4 may be defined as the angle between (1) lineconnecting diameterally opposite points on adjacent turns of thewindings 42a and Mb, in the plane containing the center lines of the twocones 3d and 36 (2) another line intersecting the axis of the cone 36 inthe same place as the first but perpendicular to the axis. in order toobtain an angle of skew such that the desired squint angle is obtainedthe angle of skew is made approximately equal to the angle which thecenter line of the conical member 36 makes with the scanning axis, thediametrical line should also be perpendicu lar to the dish axis.llnasmuch as the reduction of the squint angle occurs principally at thelonger wavelength it may be desirable only to make those turns of thewindings of larger diameter tilt at the requisite angle of skew. Thesmaller diameter of turns near the apex of the conical members then maybe equiangular. [n a particular case wherein the windings Ma and 42bwere approximately six turns each only the last three turns of thewindings were skewed.

The invention of course may be used in an antenna which does not includea dish. In that event, the apex of the conical members 34% and 36 willpoint in the direction of radiation (viz. towards the missile or spacevehicle providing the source of signals to be tracked rather thantowards a reflector and the skewed winding rotated about the center lineof the cone to provide scanning. Even if the antenna is not used as ascanning antenna, it maybe advantageously be employed in applicationwhere it is desired to have a radiation pattern over a wide band offrequencies directed at a predetermined squint angle. Thus it may beapplicable where ease of mounting is desired inasmuch as the axis of theconical member may be referenced in a particular direction with respectto the axis of the nose of an aircraft. The look angle or the directionof the beam from the antenna or the direction of radiation to which theantenna is responsive would then be determined by the squint angle whichis defined by the skew of the windings with respect to the axis of theconical member; Instead of rotating the antenna, the vehicle in whichthe antenna is mounted may be rotated in order to obtain a scanningaction.

From the foregoing description, it will be apparent that there has beenprovided an improved antenna which is particularly adapted for use as awide band scanning antenna as in telemetry applications and especiallywhere a missile or space vehicle is to be tracked. While illustrativeembodiments of the antenna have been described and other applicationsfor the antenna have been suggested, various other applications andmodifications in the herein described antenna will undoubtedly suggestthemselves to those skilled in the art. Accordingly, the foregoingdescription should be taken merely as illustrative and not in anylimiting sense.

What we claim is:

1. An antenna comprising a. an electrically conductive element spirallywound to define a substantially conical surface having an axis, and

b. at least those turns of said element having larger diameters beingskewed with respect to said axis so as to depart from equiangularrelationship with respect to the other turns of said element which arenot so skewed, adjacent ones of said larger diameter turns which aredisposed successively in a direction towards the apex of said conicalsurface being spaced from each other by progressively smaller distances.

2. The invention as set forth in claim 1 wherein the projection of saidturns having larger diameters on a plane perpendicular to said axis isan elliptical spiral.

3. The invention as set forth in claim 1 including a reflector having anaxis and a focal point spaced from said reflector along said reflectoraxis, said surface and said reflector being disposed on opposite sidesof said focal point with said reflector axis at a predetermined angle tosaid surface axis whereby the direction of the radiation pattern of saidturns of spirally wound element is substantially parallel to saidreflector axis.

4. The invention as set forth in claim 3 including means for supportingsaid elements on said surface for rotation about said reflector axis.

5. The invention as set forth in claim 3 including a second electricallyconductive element spirally wound identically with said first namedconductive element except in an opposite sense to define a secondsubstantially conical surface, the axis of said second surface beingdisposed in the same plane as said first surface axis and said reflectoraxis, said second surface axis and said first surface axis intersectingsaid reflector axis at the same point and making the same angle withsaid reflector axis as said first surface axis.

6. The invention as set forth in claim 5 including means for supportingsaid elements on said first and second surfaces for rotation about saidreflector axis.

7. An antenna comprising first and second electrically conductiveelements spirally interwound to define a substantially conical surfacehaving an axis, at least those turns of said elements having largerdiameters being skewed with respect to said axis so as to define a skewangle which is different from the skew angle of the remaining turns, theskew angle being defined as the angle formed by a first line connectingdiametrically opposite points on turns of first and second elementswhich are adjacent to each other and a line perpendicular to the axis inthe plane containing the axis and said first line.

3. The invention as set forth in claim 7 including a transmission linehaving two conductors, connected respectively to said first and secondelements at the apex of said conical surface,

9. The invention as set forth in claim 7 wherein all of the turns ofsaid first and said second conductive elements have the same skewedrelationship.

10. The invention as set forth in claim 7 wherein said first and secondelements are so wound that the intersection of a first planeperpendicular to said axis and said surface defines a circle and asecond plane intersecting said axis at the same point as said firstplane and intersecting diametrically opposite points of adjacent ones ofsaid first and second elements and said surface defines an ellipse.

llll. A unidirectional antenna for providing a radiation pattern in acertain direction comprising a. a pair of windings spirally wound as todefine a conical surface having an axis lying at a predetermined angleto said certain direction, and

b. at least a plurality of those turns of said windings having thelarger diameters being skewed with respect to said axis so that a planeperpendicular to said certain direction and intersecting diametricallyopposite points on adjacent ones of said turns of different ones of saidwindings defining a squint angle with respect to said axis said squintangle being equal to the difference between and said predeterminedangle.

12. The invention as set forth in claim 111 including a transmissionline having two conductors connected respectively to different ones ofsaid pair of windings at ends thereof near the apex of said surface, andresistive means connected between the ends of said winding near the baseof said surface.

13. The invention as set forth in claim 12 wherein said resistive meansincludes a circular conductor disposed near the base of said surface, afirst plurality of resistive elements each connected between the lastturn of one of said pair of windings and said circular conductor atpositions spaced from each other around the periphery of said circularconductor, and a second plurality of resistive elements each connectedbetween the last turn of the other of said of windings and said circularconductor at positions spaced from each other around the periphery ofsaid circular conductor.

M. The invention as set forth in claim 13 wherein the spacing of saidpluralities of resistive element is logarithmic.

15. The invention as set forth in claim llll including a second pair ofwindings spirally wound identically with said first pair of windingsexcept in the opposite sense so as to define a second conical surfacehaving a second axis, said second axis with said dish axis bisecting theangle between the axes of said surfaces.

l7 The invention as set forth in claim 16 including means for supportingsaid surfaces for rotation about the axis of said dish.

1. An antenna comprising a. an electrically conductive element spirallywound to define a substantially conical surface having an axis, and b.at least those turns of said element having larger diameters beingskewed with respect to said axis so as to depart from equiangularrelationship with respect to the other turns of said element which arenot so skewed, adjacent ones of said larger diameter turns which aredisposed successively in a direction towards the apex of said conicalsurface being spaced from each other by progressively smaller distances.2. The invention as set forth in claim 1 wherein the projection of saidturns having larger diameters on a plane perpendicular to said axis isan elliptical spiral.
 3. The invention as set forth in claim 1 includinga reflector having an axis and a focal point spaced from said reflectoralong said reflector axis, said surface and said reflector beingdisposed on opposite sides of said focal point with said reflector axisat a predetermined angle to said surface axis whereby the direction ofthe radiation pattern of said turns of spirally wound element issubstantially parallel to said reflector axis.
 4. The invention as setforth in claim 3 including means for supporting said elements on saidsurface for rotation about said reflector axis.
 5. The invention as setforth in claim 3 including a second electrically conductive elementspirally wound identically with said first named conductive elementexcept in an opposite sense to define a second substantially conicalsurface, the axis of said second surface being disposed in the sameplane as said first surface axis and said reflector axis, said secondsurface axis and said first surface axis intersecting said reflectoraxis at the same point and making the same angle with said reflectoraxis as said first surface axis.
 6. The invention as set forth in claim5 including means for suppOrting said elements on said first and secondsurfaces for rotation about said reflector axis.
 7. An antennacomprising first and second electrically conductive elements spirallyinterwound to define a substantially conical surface having an axis, atleast those turns of said elements having larger diameters being skewedwith respect to said axis so as to define a skew angle which isdifferent from the skew angle of the remaining turns, the skew anglebeing defined as the angle formed by a first line connectingdiametrically opposite points on turns of first and second elementswhich are adjacent to each other and a line perpendicular to the axis inthe plane containing the axis and said first line.
 8. The invention asset forth in claim 7 including a transmission line having twoconductors, connected respectively to said first and second elements atthe apex of said conical surface.
 9. The invention as set forth in claim7 wherein all of the turns of said first and said second conductiveelements have the same skewed relationship.
 10. The invention as setforth in claim 7 wherein said first and second elements are so woundthat the intersection of a first plane perpendicular to said axis andsaid surface defines a circle and a second plane intersecting said axisat the same point as said first plane and intersecting diametricallyopposite points of adjacent ones of said first and second elements andsaid surface defines an ellipse.
 11. A unidirectional antenna forproviding a radiation pattern in a certain direction comprising a. apair of windings spirally wound as to define a conical surface having anaxis lying at a predetermined angle to said certain direction, and b. atleast a plurality of those turns of said windings having the largerdiameters being skewed with respect to said axis so that a planeperpendicular to said certain direction and intersecting diametricallyopposite points on adjacent ones of said turns of different ones of saidwindings defining a squint angle with respect to said axis said squintangle being equal to the difference between 90* and said predeterminedangle.
 12. The invention as set forth in claim 11 including atransmission line having two conductors connected respectively todifferent ones of said pair of windings at ends thereof near the apex ofsaid surface, and resistive means connected between the ends of saidwinding near the base of said surface.
 13. The invention as set forth inclaim 12 wherein said resistive means includes a circular conductordisposed near the base of said surface, a first plurality of resistiveelements each connected between the last turn of one of said pair ofwindings and said circular conductor at positions spaced from each otheraround the periphery of said circular conductor, and a second pluralityof resistive elements each connected between the last turn of the otherof said of windings and said circular conductor at positions spaced fromeach other around the periphery of said circular conductor.
 14. Theinvention as set forth in claim 13 wherein the spacing of saidpluralities of resistive element is logarithmic.
 15. The invention asset forth in claim 11 including a second pair of windings spirally woundidentically with said first pair of windings except in the oppositesense so as to define a second conical surface having a second axis,said second axis intersecting the axis of said first conical surface andso disposed that the same plane extends diametrically through both ofsaid surfaces.
 16. The invention as set forth in claim 15 including areflector dish having a focal point along the axis of said dish and saidsurface being disposed on opposite sides of said focal point with saiddish axis bisecting the angle between the axes of said surfaces.
 17. Theinvention as set forth in claim 16 including means for supporting saidsurfaces for rotation about the axis of said dish.